Phase change cell

ABSTRACT

A phase change cell includes a housing enclosing a phase change chamber that holds a phase change material and a gas pocket. The housing includes a side wall extending between first and second end walls. A capillary is disposed in an interior surface of the side wall. In response to heating of the phase change cell, the capillary is configured to draw the phase change material in a liquid phase towards the periphery of the phase change chamber. A temperature sensor is coupled to the housing in a vicinity of the capillary to measure the phase change temperature. According to another aspect, the housing includes a moveable surface that bounds a portion of the phase change chamber. The phase change temperature of the phase change material changes based on the position of the moveable wall.

BACKGROUND

Phase change materials reliably change phase at a predeterminedtemperature to provide a repeatable temperature measurement. As heat isapplied to a solid-liquid phase change material within a phase changecell, the temperature of the phase change material in a solid phase willrises until the material reaches its phase change temperature (themelting temperature). At this point, the phase change material willcontinue to absorb a significant amount of heat at a virtually constanttemperature until all the material is transformed to the liquid phase.Likewise, the phase change material will release its stored latent heatenergy at its phase change temperature in the transition from a liquidphase to a solid phase. This characteristic flattening of thetemperature response of the phase change material during heating orcooling provides a stable, reliable indication of temperature.

High precision phase change cells are devices that encapsulate phasechange materials and yield a measurable phase change temperature duringa heating or cooling cycle. Such phase change cells can be used in avariety of contexts and environments to assist in temperaturecalibration. For example, climate systems, such as space-based climatesystems utilizing optical instruments, must be periodically calibratedin order to provide accurate data. Without regular calibration, suchinstruments are subject to temperature drift that may impact theaccuracy of the instruments. Various other climate systems requirecertain components to be accurately maintained or moved towards certaintemperatures in order for these components to be properly operated andcontrolled.

Temperature readings provided by a phase change cell during a phasechange can be used to calibrate temperature sensors. However, designs ofexisting phase change cell enclosures suffer from limitations indelivering accurate temperature measurements. Typically, the chamberinside the phase change cell containing the phase change material ispartially filled with a gas pocket. Especially in micro-gravityenvironments, the location of the gas pocket within the chamber cannotbe controlled and potentially could be adjacent to the wall of the phasechange cell where the temperature sensor is located. This situation mayresult in an inaccurate reporting of the phase change materialtemperature, because the temperature of the gas pocket is measuredinstead of that of the phase change material.

Further, in some contexts it is desirable to measure the phase changetemperature of several different materials in order to develop atemperature curve to support more accurate calibration using curvefitting. However, this technique generally requires a more complex phasechange cell structure involving multiple chambers to separately housedifferent phase change materials, together with corresponding controlsand sensors.

SUMMARY

Described herein is a phase change cell that, according to one aspect,comprises a housing enclosing a phase change chamber. The housingincludes a first end wall configured to be coupled to a heating and/orcooling source, a second end wall, and a side wall longitudinallyextending between the first and second end walls. A phase changematerial occupies a majority of a volume of the phase change chamber andis configured to change between a solid phase and a liquid phase at aphase change temperature in response to heating or cooling. A gas pocketis disposed in the phase change chamber in communication with the phasechange material. A capillary is disposed along a periphery of the phasechange chamber and comprises a channel formed in an interior surface ofthe side wall. In response to heating of the phase change cell, thecapillary is configured to draw the phase change material in a liquidphase towards the periphery of the phase change chamber. A temperaturesensor is coupled to the housing in a vicinity of the capillary. Thecapillary can be one of a plurality of capillaries each comprising achannel formed in the interior surface of the side wall.

According to another aspect of the described phase change cell, thehousing enclosing the phase change chamber includes a moveable surfacethat bounds a portion of the phase change chamber. The phase changematerial disposed in the phase change chamber has a phase changetemperature that is a function of the pressure at which the phase changematerial is maintained in the phase change chamber. A controllercontrols the moveable surface to move between a first position at whichthe phase change chamber has a first volume that causes the phase changematerial to be at a first pressure, and second position at which thephase change chamber has a second volume that causes the phase changematerial to be at a second pressure that is different from the firstpressure, such that the phase change material has first and second phasechange temperatures in response to the moveable surface being in thefirst and second positions, respectively. According to one exampleimplementation, and end wall of the housing can be a bimetallic diskthat “pops” between a substantially flat shape below a certaintemperature and a convex shape above that temperature. The controllercan be a thermoelectric cooler capable of heating and cooling,controlled by a processor to heat and/or cool the bimetallic disk inaccordance with a calibration process to perform measurements of thephase change temperature at plural different pressures.

The above and still further features and advantages of the describedsystem will become apparent upon consideration of the followingdefinitions, descriptions and descriptive figures of specificembodiments thereof wherein like reference numerals in the variousfigures are utilized to designate like components. While thesedescriptions go into specific details, it should be understood thatvariations may and do exist and would be apparent to those skilled inthe art based on the descriptions herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the outer housing of a phase changecell described herein.

FIG. 2 is a transverse cross sectional view of the phase change cellshown in FIG. 1 taken along line II-II.

FIG. 3A is a cutaway view of the phase change cell of FIG. 1illustrating the phase change material and gas pocket disposed in theinterior phase change chamber.

FIG. 3B is a cutaway view of the phase change cell of FIG. 1 with thephase change material removed to show an example configuration ofcapillaries formed in the wall of the phase change chamber.

FIG. 4 is a cutaway view of the phase change cell of FIG. 1 with thephase change material removed to show another example configuration ofcapillaries formed in the wall of the phase change chamber.

FIGS. 5A-5E are transverse cross sectional views of the side wall of aphase change cell showing several examples of cross-sectional shapes ofthe capillaries.

FIG. 6A is a longitudinal cross sectional view of a phase change cellhaving moveable surfaces that are in a first, substantially flatposition.

FIG. 6B is a longitudinal cross-sectional view of the phase change cellof FIG. 6A wherein the moveable surfaces are in a second, bowedposition.

FIG. 7 is a flow diagram illustrating operations performed to measuremultiple phase change temperatures with a single phase change cell indifferent volume configurations.

DETAILED DESCRIPTION

One aspect of the described phase change cell involves introducingcapillary structures on the inner surface of the wall of the chamberthat houses the phase change material. The narrow channels of thecapillaries enable capillary action to draw the phase change material ina liquid phase towards the wall and to prevent a gas pocket within thechamber from being positioned along the wall. In this manner, thetemperature measured by a sensor disposed in the vicinity of a capillaryaccurately reflects the temperature of the phase change material withinthe chamber rather than a gas pocket within the chamber.

A phase change cell 10 according to a first embodiment is shown inperspective in FIG. 1. Phase change cell 10 includes a generallytube-shaped housing 12 comprising a first (e.g., bottom) end wall 14, asecond (e.g., top) end wall 16, and a side wall 18 longitudinallyextending between the first and second end walls 14, 16. In thisexample, the first and second end walls 14, 16 are substantially flat,circular disk-shaped end caps extending transversely to the longitudinalaxis of phase change cell 10. Side wall 18 is cylindrically shaped witha circular transverse cross section and extends from end to end in thedirection of the longitudinal axis. Side wall 18 is secured on its twolongitudinal ends to first and second end walls 14, 16 to form a fullyenclosed and sealed phase change chamber in the interior of phase changecell 10. A temperature sensor 20, such as a thermocouple or athermistor, is attached to the outer surface of housing 12 at a pointalongside wall 18. For convenience, temperature sensor 20 is shown inthe figures as being attached to the outer surface of housing 12.However, temperature sensor 20 can be disposed on the inner surface ofside wall 18, i.e., in the interior phase change chamber 22 withinhousing 12, or embedded into the material of housing 12. Thus, as usedherein and in the claims, the term “coupled to the housing” encompassesthe temperature sensor being positioned on the interior surface of thehousing, the exterior surface of the housing, or embedded within thehousing material.

The circular end walls 14, 16 and side wall 18 of housing 12 have anouter diameter D₁, and side wall 18 has a length (or height) h in thelongitudinal direction. By way of a non-limiting example, outer diameterD₁ can be approximately 1 inch (about 2.5 cm) and length h can beapproximately 2 inches (about 5 cm). While side wall 18 is cylindricalin the example shown in FIG. 1, more generally, the side wall can haveany of a variety of ring-like transverse cross-sectional shapes,including: oval, elliptical, regular or irregular polygonal, andcombinations of arcs, curved, and straight segments (e.g., the side wallcan have multiple flat and/or curved faces). Further, the transversecross-sectional area of the housing 12 need not be constant over thelength of housing 12. Thus, as used herein and in the claims, atube-shaped housing comprises a hollow body that extends along alongitudinal axis and that is enclosed at its two ends, the hollow bodyhaving any of a variety of ring-shaped transverse cross sectionalshapes, including but not limited to those listed above. While both endwalls are depicted as flat, circular disks in the embodiment shown inFIG. 1, the end walls can have any suitable shape that encloses thetubular housing. For example, if the side wall is shaped as an elongatedhexagonal tube, the end walls can be flat (planar) hexagonal disks. Moregenerally, the end walls need not be flat or planar. For example, thetop end wall can be dome shaped and can be formed integrally with theside wall.

First (bottom) end wall 14 is configured to be coupled to athermoelectric cooler (not shown) or other heating/cooling mechanismcapable of heating and/or cooling phase change cell 10 to cause a phasechange material within housing 12 to undergo a phase change between theliquid and solid phases (melt or solidify). As used herein and in theclaims, a heating and/or cooling source includes devices capable ofheating housing 12 (e.g., a resistance heater), devices capable ofcooling housing 12 (e.g., a refrigeration mechanism), and devicescapable of both heating and cooling housing 12 (e.g., a thermoelectriccooler). For example, at least a portion of first end wall 14 can beflat to provide good surface area contact with a thermoelectric cooler.It will be appreciated that the contour of the outer surface of firstend wall 14 can be shaped to correspond to other surface shapes of aheating/cooling mechanism. Optionally, second (top) end wall 16 can alsobe configured to be coupled to a heating/cooling source. Note thattemperature sensor 20 is positioned on side wall 18 at a significantdistance from first end wall 14, e.g., on the upper half of side wall 18to ensure that the temperature reading is not unduly influenced by theheating and/or cooling source.

Housing 12 comprises a material that is highly thermally conductive suchthat heating or cooling of the housing is readily conveyed throughhousing 12 to the phase change material contained in the phase changechamber inside housing 12. The material of housing 12 should also benon-reactive with the phase change material and any gases within thephase change chamber. By way of non-limiting examples, housing 12 can bemade of stainless steel, aluminum, Teflon, beryllium, H20, or (NIST)ITS-90 temperature scale materials or combinations thereof.

Any of a variety of manufacturing techniques can be used to make andassemble housing 12 of phase change cell 10 including extrusion, molds,and three-dimensional (3D) printing. Where end walls 14, 16 and sidewall 18 are manufactured as separate components, any of a variety ofmechanisms can be used to join end walls 14, 16 to side wall 18,including but not limited to: welding, epoxy, solder, screws, threadedsurfaces, interlocking surfaces, and combinations thereof.

FIG. 2 is a transverse cross-sectional view of the phase change cell 10shown in FIG. 1 taken along the line II-II. The interior phase changechamber 22 within housing 12 is air tight such that, under operationalconditions, no solid, liquid, or gas contained in phase change chamber22 can leak out of housing 12, and no solid, liquid, or gas can enterphase change chamber 22 from the exterior of housing 12. The innerdiameter D₂ of housing 12 constitutes the diameter of phase changechamber 22. By way of a non-limiting example, the inner diameter D₂ canbe approximately 0.9 inch (about 2.25 cm), and the cross-sectionalthickness t of side wall 18 (in a radial direction) can be approximately0.1 inch (about 2.5 mm).

A phase change material 24 is contained in phase change chamber 22 andfills the majority of the volume of phase change chamber 22. Forexample, phase change chamber 22 is at least 75% filled with phasechange material 24. According to another example, phase change chamber22 is as least 90% filled with phase change material 24. According toanother example, phase change chamber 22 is at least 95% filled withphase change material 24. Phase change material 24 is capable ofchanging between the solid and liquid phases upon heating or cooling ofan outer surface of housing 12. Phase change material 24 can be any of awide variety of substances with a relatively high heat of fusion that,when melting and solidifying at a certain temperature (the phase changetemperature), is capable of storing and releasing significant amounts ofenergy. For example, phase change material can be a metal with arelatively low melting temperature, such as gallium, gallium selenium,gallium indium, gallium tin, or mercury.

As a result of its phase change properties, when heat is applied tophase change material 24 in the solid phase, its temperature rises untilit begins to melt. As heat continues to be applied, the temperature ofphase change material 24 remains steady at the phase change temperatureuntil it is completely melted. Once melted, further heating of phasechange material 24 causes its temperature to increase. Likewise, whencooling is applied to phase change material 24 in the liquid phase, itstemperature decreases until it begins to solidify. As cooling continuesto be applied, the temperature of phase change material 24 remainssteady at the phase change temperature until it is completelysolidified. This characteristic response of phase change material 24allows the phase change temperature to be readily measured during aphase change from solid to liquid or from liquid to solid. The phasechange temperature measured by temperature sensor 20 can be used in avariety of ways to calibrate sensors and maintain a specific temperatureof a component, an enclosure, or a system environment that employs phasechange cell 10.

Since phase change material 24 may slightly expand or contract duringphase changes, to avoid significant increases in internal pressure inchamber 22 and undesirable stress on sealing joints of housing 12 and tomaintain a controlled pressure, phase change chamber 22 is alsopartially filled with a gas 26. Gas 26 is selected to be non-reactivewith housing 12 and phase change material 24. By way of non-limitingexamples, gas 26 is an inert gas such as nitrogen or a noble gas such ashelium or argon, or combinations thereof. In general gas 26 is much morecompressible than phase change material 24.

Within phase change chamber 22, at least some of gas 26 may coalesceinto a bubble or pocket of gas of a significant size. An example of sucha gas pocket is shown in FIG. 2. FIG. 3A is a cutaway view of phasechange cell 10 illustrating the interior of phase change chamber 22 inwhich gas pocket 26 is in communication with phase change material 24(e.g., gas pocket 26 is entirely surrounded by phase change material 24in this example). Particularly in a zero gravity or microgravityenvironment, such a gas pocket can form and float anywhere within phasechange material 24 (in a liquid phase) inside phase change chamber 22.For example, a pocket of gas 26 could potentially drift to the edge ofphase change chamber 22 and come into contact with the inner surface ofside wall 18 adjacent to the position of temperature sensor 20. In thiscase, the temperature measured by temperature sensor 20 may at leastpartially reflect the temperature of the gas pocket instead of phasechange material 24 itself. Since the temperature of the gas pocket canbe significantly different from that of phase change material 24, havingthe gas pocket positioned against the inner surface of housing 12 in thevicinity of sensor 20 could potentially cause an inaccurate measurementof the phase change temperature of phase change material 24 contained inchamber 22.

To prevent a gas pocket from contacting the inner surface of housing 12(the perimeter of phase change chamber 22), capillaries 28 are formedalong the interior surface of side wall 18, as shown in FIG. 2. FIG. 3Bis the same cutaway view of the phase change cell 10 as FIG. 3A but withthe phase change material 24 removed to show an example configuration ofcapillaries 28 formed in the interior surface of side wall 18 at theperiphery of phase change chamber 22. In particular, in this example,the capillaries comprise channels in the interior surface of side wall18 such that each capillary is formed by the removal or absence of aportion of side wall 18. In the transverse cross-section shown in FIG.2, each capillary 28 has a width in the circumferential direction and adepth in the radial direction. That is, the capillaries 28 (channels)extend outward (beyond the base inner diameter D₂ of side wall 18) andinto side wall 18 such that the radial thickness of side wall 18 issomewhat diminished at the location of each capillary 28.

For ease of illustration, only ten capillaries are shown in FIGS. 2 and3B and the width and depth dimensions of the capillaries are notnecessarily to scale in the figures relative to the size of housing 12.In practice, any suitable number of capillaries can be formed in theinterior surface of side wall 18. In the example shown in FIG. 3B, eachcapillary 28 extends along the entire length of side wall 18 from firstend wall 14 to second end wall 16. Optionally, the capillaries formed inside wall 18 can further extend along the interior surfaces of first endwall 14 and/or second end wall 16. This arrangement is shown in FIG. 3Bfor first (bottom) end wall 14. According to another option, thecapillaries may extend along only a portion of side wall 18, i.e., notthe entire length of side wall 18 from the first end wall 14 to thesecond end wall 16. While the capillaries shown in the figures have anopen construction along their length (i.e., having an opening to thephase change chamber 22), according to another option, the capillariescan have a closed, tube-like construction along at least a portion oftheir extent, with openings to the phase change chamber 22 at the endsof the capillaries.

In operation, capillaries 28 employ capillary action to draw phasechange material 24 in a liquid phase into the narrow channels. Capillaryaction is the ability of a liquid to flow in narrow spaces without theassistance of external forces such as gravity as a result ofintermolecular forces between the liquid and surrounding solid surfaces.If the cross-sectional area of the capillaries (channels) issufficiently small, then the combination of surface tension and adhesiveforces between the liquid and channel surfaces act to draw and lift theliquid.

The shape of housing 12 and the orientation of capillaries 28 facilitatethe process of drawing the phase change material 24 in liquid phase intoand through capillaries 28. Because the tube-shaped housing 12 isdesigned to be heated (and/or cooled) at one end, phase change cell 10can behave in the manner of a heat pipe in terms of fluid flow.Specifically, as heat is applied to first end 14, a temporary heat flux(temperature differential) is created from the top end (cool) to thebottom end (hot) of phase change cell 10. In response to heating ofphase change cell 10, capillaries 28 draw phase change material 24 in aliquid phase towards the periphery of phase change chamber 22.Specifically, as phase change material 24 begins to melt, it becomes aworking fluid capable a conveying heat energy from the heated first endto the cooler second end. As suggested by the directional arrows inFIGS. 2 and 3A, the wicking action of capillaries 28, which extend fromfirst end wall 14 to second end wall 16, draws the liquid phase changematerial 24 towards the interior surface of housing 12 (i.e., the outerwall of phase change chamber 22). The end-to-end temperaturedifferential causes the liquid phase change material 24 to travel upthrough capillaries 28, away from heated first end wall 14 and towardssecond end wall 16, as suggested by the directional arrows in FIG. 3B.

With a sufficient number of spaced-apart capillaries 28, the liquidphase change material 24 contacts the entire interior surface of sidewall 18, forcing any gas pockets towards the center of phase changechamber 22 and away from side walls 18. In this manner, regardless ofwhere temperature sensor 20 is place along the circumference (perimeter)of the outer surface of side wall 18, temperature sensor 20 will be inthe vicinity of one or more capillaries 28, thereby ensuring thattemperature sensor 20 is measuring the temperature of phase changematerial 24 rather than the temperature of a gas pocket 26. Thus, asused herein and in the claims, the term “vicinity” refers to thetemperature sensor being sufficiently close to a capillary to ensurethat, when the phase change material in the liquid phase is being drawntowards and through the capillary, the temperature sensor is adjacent tothe phase change material being drawn by the capillary and not adjacentto a gas pocket.

The cross-sectional dimensions of capillaries 28 must be sized toprovide sufficient capillary action to pull the liquid phase changematerial into the channels and to lift the liquid phase change materialthrough the channels from one end of phase change cell 10 to the otherend in response to the end-to-end temperature differential. Suchdimensions depend in part on the viscosity of the liquid phase changematerial 24. By way of a non-limiting example, the cross-sectional depthand width dimensions can be on the order of 0.05 to 0.1 inch (on theorder of 1 to 2 mm), also depending in part on the thickness of housing12.

Minimally, a single capillary can be formed on the inner surface ofhousing 12, provided the location of the single capillary is closelyaligned with the position of temperature sensor 20. However, bydistributing a plurality of capillaries along the perimeter of phasechange chamber 22, precise placement of temperature sensor 20 becomesless critical, since with a sufficient number of capillaries,temperature sensor 20 will necessarily be in the vicinity of one or moreof the capillaries, thereby simplifying the manufacturing process.Further, having a distributed array of capillaries enables use ofmultiple temperature sensors along the perimeter of housing 12 withoutthe risk of a gas pocket being adjacent to any of the sensors. Accordingto one example, the spacing between adjacent capillaries can beapproximately the same as the circumferential width of the capillaries,such that about half of side wall 18 constitutes the capillary channels.

While capillaries 28 shown in FIG. 3B extend linearly in thelongitudinal direction alongside wall 18 between the first and secondend walls 14, 16, it will be appreciated that the capillaries can haveother configurations, provided the flow direction of the capillaries hasat least some component in the longitudinal direction to allow theend-to-end heat flux to draw the liquid phase change material 24 throughthe capillaries. For example, as shown in FIG. 4, capillaries 28′ canextends in a curved or helical (twist) pattern alongside wall 18 betweenfirst and second end walls 14, 16. As with the previous drawings, only afew capillaries 28′ are shown for ease of illustration, and thecapillary dimensions and spacings are not necessarily to scale. Otherpossible capillary patterns include: a wavy or sinusoidal path, azig-zag path, a meandering path, and a pseudo random path having adirection with a continuous longitudinal component. Further, thecapillaries can be arranged to intersect, e.g., intersecting clockwiseand counterclockwise helical patterns forming a diamond-shaped grid.

While capillaries 28 and 28′ shown in FIGS. 2, 3A, 3B, and 4 comprisechannels having a substantially rectangular cross-sectional shape, moregenerally, the capillary channels can have any cross-sectional shapesuitable for drawing the liquid phase change material into the channels.For example, as shown in FIG. 5A, capillaries 28 can have a rectangularcross-sectional shape with a greater depth (in the radial direction)than width (in the circumferential direction). As shown in FIG. 5B,capillaries 28 can have a substantially trapezoidal cross section, e.g.,with the circumferential width increasing with increasing radial depthinto side wall 18. The cross-sectional shape of the capillaries can alsobe rounded, curved, or arched. For example, in FIG. 5C, capillaries 28have a cross-sectional shape of most of a circle, such that the diameterof each capillary 28 initially increases as the radial depth increasesand then diminishes in accordance with the shape of the circle. In FIG.5D, the circumferential width of capillaries 28 is initially constantwith increasing depth, but then decreases in accordance with the shapeof semi-circle (partial race track shape). Finally, in FIG. 5E,capillaries 28 have an hour glass cross-sectional shape.

It will be appreciated that not all of the capillaries need to have thesame cross-sectional shape or the same size or spacing from adjacentcapillaries. Further, individual capillaries can vary in cross-sectionalshape and size over their longitudinal extent to facilitate flow of theliquid phase change material (e.g., a capillary can become wider ornarrower as it extends from one end to the other).

Because the capillaries extend along the length of the side wall, wherethe tubular side wall body is manufactured using extrusion techniques,it is possible to directly form the capillaries during the extrusionprocess rather than the alternative of subsequently machining anotherwise smooth interior surface of the tube. The capillaries can alsobe formed using the molding and 3D printing manufacturing techniquespreviously described.

As previously explained, it may be desirable to measure severaldifferent phase change temperatures (e.g., using different phase changematerials) so that a curve of phase change temperatures can be developedto support more accurate calibration. However, this technique generallyrequires a more complex phase change cell structure involving multiplechambers to separately house different phase change materials, togetherwith corresponding controls and sensors.

In accordance with another aspect of the described phase change cell,the cell housing includes a moveable surface that bounds a portion ofthe phase change chamber. By moving the moveable surface from oneposition to another position, the volume of the phase change chamber canbe changed. According to the ideal gas law, PV=nRT, (where P is the gaspressure, V is the gas volume, n is the amount of gas, R is theuniversal gas constant, and T is the gas temperature), since the mass ofthe phase change material and the amount of gas within the phase changechamber is constant, at a given temperature, if the volume of the gas isdecreased by decreasing the volume of the phase change chamber, then thepressure P of the gas (and hence the pressure within the phase changechamber) increases. Conversely, if the volume of the gas is increased byincreasing the volume of the phase change chamber, then the pressure Pof the gas (and hence the pressure within the phase change chamber)decreases. Thus, a change in volume of the phase change chamber resultsin an inversely proportional change in pressure on the phase changematerial. Since the phase change temperature of a phase change materialis a function of pressure (the phase change temperature (melting point)decreases with increasing pressure), moving the moveable surface resultsin a different phase change temperature.

A controller controls the moveable surface to move between a firstposition at which the phase change chamber has a first volume thatcauses the phase change material to be at a first pressure, and secondposition at which the phase change chamber has a second volume thatcauses the phase change material to be at a second pressure that isdifferent from the first pressure such that the phase change materialhas first and second phase change temperatures in response to themoveable surface being in the first and second positions, respectively.By cycling through phase changes of the phase change material at two ormore pressure levels, and plurality of different phase changetemperatures can be measured by a temperature sensor using only onephase change material in a single phase change chamber whose volume isvariable. Thus, this phase change cell pressure control system avoidsthe need for multiple chambers and multiple different phase changematerials to obtain multiple phase change temperature readings.

According to one implementation, the moveable surface of phase changechamber is a surface of a bimetallic sheet having a first shape when itstemperature is below a shape-changing or “transition” temperature and asecond shape when its temperature is above the transition temperature. Abimetallic sheet comprises two layers respectively composed of twodifferent metals with different coefficients of thermal expansion. Byway of non-limiting examples, the two layers can respective comprisesteel and aluminum, steel and cooper, or steel and brass. Below acertain transition temperature, the bimetallic sheet has a certainshape, such as a flat or planar contour. As heat is applied to thebimetallic sheet, the two layers begin to expand at different rates, butthis initially causes minimal deformation. As the bimetallic sheetreaches the transition temperature, it suddenly deforms, i.e., it “pops”or snaps into a second shape, such as a bowed or convex shape, andremains in that shape above the transition temperature. By lowering thetemperature of the bimetallic sheet below the transition temperature, itsnaps back to the first shape. Thus, the bimetallic sheet assumes twodiscrete shapes depending on its temperature.

By constructing one or more of the surfaces of the housing from abimetallic sheet, the volume of the phase change chamber can be changedin a controllable manner between different discrete volumes. FIGS. 6A isa longitudinal cross-sectional side view in elevation of an exampleimplementation of a phase change cell 60 with two bimetallic surfaces,enabling the phase change cell to have several different volume/pressureconfigurations. While not shown in FIGS. 6A and 6B for ease ofillustration, it will be appreciated that the capillary mechanismdescribed in connection with FIGS. 1-5E can be employed along with thedescribed volume control mechanisms in the same phase change cell.Similar to the phase change cell shown in FIG. 1, phase change cell 60includes a generally tube-shaped (e.g., cylindrical) housing 62comprising a first (e.g., bottom) end wall 64, a second (e.g., top) endwall 66, and a side wall 68 longitudinally extending between the firstand second end walls 64, 66. In this example, each of first and secondend walls 64, 66 comprises a circular bimetallic disk. By way of anon-limiting example, the bimetallic disks can have a thickness ofapproximately one to five hundredths of an inch (about 0.25 to 1.25 mm).The bimetallic disks can be affixed to side wall 18 using, for example,computer controlled electron beam welding or resistance welding.

In FIG. 6A, the bimetallic disk end walls 64, 66 are in their respectivefirst positions and are substantially flat disks. Side wall 68 issecured on its two longitudinal ends to the first and second bimetallicdisk end walls 64, 66 to form a fully enclosed and sealed phase changechamber in the interior of phase change cell 10. A temperature sensor70, such as a thermocouple or a thermistor, is coupled to housing 72 ata point along side wall 78. Here again, while temperature sensor 70 isshown on the exterior surface of housing 72 for convenience, temperaturesensor 70 can be disposed on an inner surface of housing 72 or embeddedwithin the material of housing 72. A phase change material 74 iscontained in phase change chamber 72 and fills the majority of thevolume of phase change chamber 72. Phase change chamber 72 is alsopartially filled with a gas 76. Phase change material 74 and gas 76 canhave the properties and compositions previously described in connectionwith implementation shown in FIGS. 2 and 3A.

A controller 80 is coupled to bimetallic disks 74, 76 and is configuredto independently control the temperatures of the bimetallic disks tocause the disks to switch between two discrete shapes. For example,controller 80 can comprise two thermoelectric coolers respectivelycoupled to the first and second bimetallic disk end walls 64, 66, and aprocessor that controls the thermoelectric coolers in accordance with amulti-cycle phase change sequence.

The processor of controller 80 essentially performs certain operationsto carry out a multi-cycle phase change sequence and can be implementedin hardware, software, or a combination of hardware and software, asappropriate. For example, the processor can include one or moremicroprocessors, microcontrollers, or digital signal processors capableof executing program instructions (i.e., software) for carrying out atleast some of the various operations and tasks to be performed bycontroller 80. Controller further includes one or more memory or storagedevices to store a variety of data and software instructions (controllogic) for execution by the processor. The memory may comprise read onlymemory (ROM), random access memory (RAM), magnetic disk storage mediadevices, optical storage media devices, solid-state memory devices,flash memory devices, electrical, optical, or other physical/tangible(e.g., non-transitory) memory storage devices. Thus, in general, thememory comprises one or more tangible (non-transitory)processor-readable or computer-readable storage media that stores or isencoded with instructions (e.g., control logic/software) that, whenexecuted by the processor, cause the processor to perform the operationsdescribed herein below. One or more of the components of controller 80can also be implemented in hardware as a fixed data or signal processingelement, such as an application specific integrated circuit (ASIC) thatis configured, through fixed hardware logic, to perform certainfunctions. Yet another possible processing environment is one involvingone or more field programmable logic devices (e.g., FPGAs), or acombination of fixed processing elements and programmable logic devices.

FIG. 6B shows the first and second bimetallic disk end walls 64, 66 intheir respective second shapes in which the disks are bowed inward(convex on the side facing phase change chamber 72) to reduce the volumeof phase change chamber 72. In this case, controller 80 applies heat viarespective first and second thermoelectric coolers (not shown) to firstand second bimetallic disks 64, 66. Once the bimetallic disks are heatedpast their transition temperature, they deform to the bowed shape,thereby moving to their respective second positions. By separatelycontrolling the two bimetallic disks, three different volumeconfigurations and three corresponding phase change temperatures can beachieved. Specifically, a first, largest volume results from controllingboth bimetallic disks 64, 66 to be in their respective firstshapes/positions as shown in FIG. 6A (i.e., both disks are flat). Asecond, smaller volume results from controlling one of the bimetallicdisks 64, 66 to be in its first, flat shape/position and the other ofthe bimetallic disks to be in its second, bowed shape/position. A third,smallest volume results from controlling both bimetallic disks 64, 66 tobe in their second, bowed shape/position.

Note that is possible to make other surfaces of phase change chamber 72controllable via this same bimetallic mechanism. For example, side wall18 can be constructed from a bimetallic sheet shaped into a cylinder ina first shape/position. Upon heating side wall 18 above its transitiontemperature, it deforms into a hyperboloid shape, thereby reducing thevolume of phase change chamber 72. While the example shown in FIGS. 6Aand 6B employ bimetallic sheets (disks) that bow inward, it will beappreciated that a bimetallic sheet can be oriented relative to thephase change chamber to bow outward instead of inward to effect a volumechange.

The temperature of the bimetallic sheet(s) can be controlled as part ofthe heating or cooling of the phase change material. Alternatively, thetemperature of the bimetallic sheet(s) can be controllable independentof the heating or cooling of the phase change material (e.g., differentthermoelectric coolers can be responsible for heating and cooling thephase change material and a bimetallic sheet). Depending on whether theheating or cooling of the bimetallic sheet(s) is incorporated into theheating or cooling of the phase change material, measures can beemployed to provide some degree of thermal isolation between thebimetallic surfaces and the phase change material (e.g., an insulatinglayer on the interior surface of the bimetallic sheet(s)).

In addition to the bimetallic sheet mechanism, any of a wide variety ofother volume control mechanisms can be employed to provide one ormoveable surfaces within a phase change chamber to change the volumebetween discrete states. For example, piezoelectric surfaces that deformin response to an electric current or electric field can be placedwithin the phase change chamber to modify its volume. According toanother option, one or more of the housing walls can be mechanicallymoved relative to the other walls using an actuator, servomotor, or thelike. According to yet another option, the volume of the phase changechamber can be modified by selective inserting and withdrawing an objector mass into the chamber. For example, a screw mechanism can be used tocontrol the extent to which an object protrudes into the chamber fromone of the housing walls. According to still another option, ahydro-mechanical device can be disposed in the phase change chamber. Forexample, the hydro-mechanical device can comprise an encapsulated regionof fluid whose properties can be used to change the volume of the phasechange chamber.

FIG. 7 is a flow chart 700 illustrating operations performed to measuretwo different phase change temperatures using a single,volume-controlled phase change cell. Initially, in operation 710, amoveable surface of a phase change chamber is placed in a firstposition, resulting in the phase change chamber have a first volume V₁and a first pressure P₁ that is inversely proportional to volume V₁. Inoperation 720, heating or cooling is applied to the phase change cell(e.g., via a thermoelectric cooler) to cause a first phase change of thephase change material from the solid phase to the liquid phase (melting)or from the liquid phase to the solid phase (solidifying). As usedherein and in the claims, a phase change “between the solid phase andthe liquid phase” can be either melting or solidifying. During the firstphase change, the phase change temperature T₁ of the phase changematerial is measured by a temperature sensor in operation 730.

In operation 740, the moveable surface of the phase change chamber ismoved to a second position, resulting in the phase change chamber have asecond volume V₂ (V₂≠V₁) and a second P₂ (P₂≠P₁) that is inverselyproportional to volume V₂. Preferably, the phase change material is in aliquid phase while the moveable surface is moved to minimize theresistance to moving. In operation 750, heating or cooling is applied tothe phase change cell (e.g., via a thermoelectric cooler) to cause asecond phase change of the phase change material from the solid phase tothe liquid phase (melting) or from the liquid phase to the solid phase(solidifying). If the phase change material is in a liquid state whilethe moveable surface is moved, and a melting phase change is used forthe second phase change, an intervening operation of re-solidifying thephase change material is required. During the second phase change, thephase change temperature T₂ (T₂≠T₁) of the phase change material ismeasured by the temperature sensor in operation 760.

Additional phase change temperatures can be measured by further changingthe volume of the phase change chamber. For example, the moveablesurface can be moved to a third position resulting in the phase changechamber have a third volume V₃ (V₃≠V₁ or V₂) and a third pressure P₃(P₃≠P₁ or P₂) that is inversely proportional to volume V₃, and the phasechange temperature T₃ is measured during a third phase change. Accordingto another option, another moveable surface that bounds the phase changechamber can be moved to produce the third volume. For example, themoveable surfaces can surfaces of two different bimetallic sheets, suchas bimetallic disks forming the top and bottom walls of a cylindricalphase change cell. In general, any practical number of discrete phasechange chamber pressure levels and corresponding phase changetemperatures can be achieved by controlling the position of one moveablesurface or the positions of plural moveable surfaces in concert tocreate different discrete volumes within the same phase change chamber.In this manner, a simplified phase change cell design can be used toacquire a temperature calibration curve having several different phasechange temperatures.

Having described example embodiments of a phase change cell, it isbelieved that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims. Although specificterms are employed herein, they are used in a generic and descriptivesense only and not for purposes of limitation.

What is claimed is:
 1. A phase change cell, comprising: a tube-shapedhousing enclosing a phase change chamber, the housing including a firstend wall configured to be coupled to a heating and/or cooling source, asecond end wall, and a side wall having an interior surface andlongitudinally extending between the first and second end walls, whereinsaid phase change chamber is sealed air tight such that under operatingconditions no solid, liquid or gas can egress from or enter the chamber;a solid phase change material occupying a majority of a volume of thephase change chamber, the phase change material being configured tochange between a solid phase and a liquid phase at a phase changetemperature in response to heating or cooling; a gas pocket disposed inthe phase change chamber in communication with the phase changematerial, wherein said gas is non-reactive with said phase changematerial and is movable within the phase change material when the phasechange material is in said liquid phase; a capillary disposed along aperiphery of the phase change chamber, the capillary comprising achannel formed in the interior surface of the side wall and extendingfrom the first end wall to the second end wall, wherein in response toheating of the phase change cell, the capillary is configured to drawthe phase change material in a liquid phase towards the periphery of thephase change chamber; and a temperature sensor for measuring thetemperature of the phase change material coupled to an exterior surfaceof the housing in a vicinity of the capillary.
 2. The phase change cellof claim 1, wherein the capillary is one of a plurality of capillarieseach comprising a channel formed in the interior surface of the sidewall and extending from the first end wall to the second end wall. 3.The phase change cell of claim 2, wherein the capillaries further extendalong an interior surface of the first end wall and/or the second endwall, such that the capillaries are configured to draw the phase changematerial in the liquid phase towards the periphery throughout the phasechange chamber, forcing the gas pocket to remain in an interior regionof the phase change chamber, entirely surrounded by the phase changematerial.
 4. The phase change cell of claim 1, wherein, in response toheating the first end wall, the phase change material is drawn throughthe capillary in a direction of the second end wall.
 5. The phase changecell of claim 1, wherein the phase change material comprises metal. 6.The phase change cell of claim 1, wherein the capillary extends linearlyin a longitudinal direction along the side wall between the first andsecond end walls.
 7. The phase change cell of claim 1, wherein thecapillary extends in a helical pattern along the side wall between thefirst and second end walls.
 8. The phase change cell of claim 1, whereinthe gas pocket occupies less than 25% of the volume of the phase changechamber.